In distillations, absorption, gas-scrubbing and like operations, columns are widely used which include baffling structures to promote the heat exchange and mass transfer between a liquid phase and a gas phase. These structures can be trays or the like, for example, bell type or bubble trays, sieve trays and valve trays, or the structures can be randomly arranged packing bodies like, for example, Raschig rings, Pall rings or saddle-shaped bodies, or the structures can be ordered or structured packings like, for example, plates composed of sheet metal with corrugations or folds.
The ordered or structured packing can have geometries providing a cross channel structure or cross flow channels. Such structured packings are now widely used. As a general matter, the packed columns have a maximum capacity for heat exchange and/or mass transfer, measured by the throughput of one or both of the fluids which is limited by the commencement of flooding. This applies as well to cross-flow channel structured packings. Visual monitoring of flooding in distillation columns which may be equipped with structured packing in a cross channel system have shown that the flooding appears to start at lower edges of individual packing layers. To increase the efficiency of the mass or heat exchange or the capacity of such packings, efforts have been made to so modify the lower edges and partially, where appropriate, upper edges of individual packing layers, to increase the throughputs of liquid and gas through the packing in these regions.
For example, EP 858 830 A1 provides an increase in the spacing between the neighboring packing elements at the lower portion of the packing layers, especially at the lower edges by reducing in these regions the bend height. Further possibilities have been described as well, for example, partial or complete removal of part of the material of the packing, for example, removal of a portion of every second packing element at the lower ends of the packing layers.
In WO 97/16247 an alternative has been described in which the geometry of the cross channel structure is altered. The bends or folds no longer are straight as in earlier cross channel structures but are given an S shape and are so arranged that the bend or fold direction at the lower and upper ends of the packing layers run substantially vertically.
DE 100 01 694 A1 describes a similar packing in which the folds or bends only at the lower end are redirected toward the vertical.
When the packings have been fabricated from sheet metal in the past, independently of the geometries described above, efforts were made to keep the costs down by reducing the wall thickness of the packing sheets or plates. At the present time, the various manufacturers of such cross-channel structured packings have tended to use sheets or plates which have been reduced in wall thickness to 0.075 to 0.1 mm from sheet thicknesses of 0.15 to 0.2 mm as were utilized in earlier periods. Both in the earlier periods and at the present, the packings were composed of individual layers, plates or sheets of the same wall thickness over the entire packing bed.
Advantageously, the reduction in the uniform wall thickness has resulted in reduced material cost and a reduced weight of the baffling in the column. However, with that reduction in the wall thickness there has been some reduction in the mechanical stability of the packing and some limitations as to the packing height which can be used since there is a danger of mechanical deformation of the packing in mounting in the column. Undesired deformation can detrimentally effect the distribution of liquid in the packing giving rise to stranding and interfering with turbulence with an end result of a loss in efficiency.